1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device, and to a method for fabricating one. More particularly, the present invention relates to a nitride semiconductor light-emitting device that uses a nitride semiconductor as a substrate thereof
2. Description of Related Art
There have been fabricated prototypes of semiconductor laser devices that oscillate in a region ranging from ultraviolet to visible light by the use of a nitride semiconductor material as exemplified by GaN, AlN, InN, and composite crystals thereof. For such purposes, GaN substrates are typically used, and therefore GaN substrates have been intensively researched by a host of research-and-development institutions. At the moment, however, no semiconductor laser devices offer satisfactorily long useful lives, and accordingly what is most expected in them is longer useful lives. It is known that the useful life of a semiconductor laser device strongly depends on the density of defects (such as vacancies, interstitial atoms, and dislocations, all disturbing the regularity of a crystal) that are present in a GaN substrate from the beginning. The problem here is that substrates with low defect density, however effective they may be believed to be in achieving longer useful lives, are extremely difficult to obtain, and therefore researches have been eagerly done to achieve as much reduction in defect density as possible.
For example, Applied Physics Letter, Vol. 73 No. 6 (1998), pp. 832-834, reports fabricating a GaN substrate by the following procedure. First, on a sapphire substrate, a 2.0 μm thick primer GaN layer is grown by MOCVD (metalorganic chemical vapor deposition). Then, on top of this, a 0.1 μm thick SiO2 mask pattern having regular stripe -shaped openings is formed. Then, further on top, a 20 μm thick GaN layer is formed again by MOCVD. Now, a wafer is obtained. This technology is called ELOG (epitaxially lateral overgrown), which exploits lateral growth to reduce defects.
Further on top, a 200 μm thick GaN layer is formed by HVPE (hydride vapor phase epitaxy), and then the sapphire substrate serving as a primer layer is removed. In this way, a 150 μm thick GaN substrate is produced. Next, the surface of the obtained GaN substrate is ground to be flat. The thus obtained substrate is known to have a defect density as low as 106 cm−2 or less.
It has been found, however, that, even with a semiconductor laser device fabricated by growing, by a growing process such as MOCVD, a nitride semiconductor film on a low-defect substrate, such as the one obtained by the above-described procedure, it is still impossible to obtain a useful life that is satisfactorily long for practical use. Through an intensive study in search of the reason for that, the inventors of the present invention have found out that strains and cracks present inside a nitride semiconductor film greatly affect the deterioration and yield rate of a semiconductor laser device. Even when a GaN substrate that is homoepitaxial with a nitride semiconductor film is used, the grown nitride semiconductor film includes layers of InGaN, AlGaN, and the like whose lattice constants and thermal expansion coefficients differ from those of GaN. The presence of these layers different from GaN causes an InGaN active layer and other layers to receive compressive stress. It has been found that the resulting strains present inside the film accelerate the deterioration of the semiconductor laser device.
Another problem with a nitride semiconductor film is many cracks that develop therein, resulting in a low yield rate. The development of such cracks is also greatly affected by strains present inside the film.
More specifically, when a laser structure formed of a nitride semiconductor thin film is epitaxially grown on a nitride semiconductor substrate, many cracks (for example, several or more within a width of 1 mm) develop, resulting in an extremely low yield rate of devices with the desired characteristics. If a fabricated device contains cracks, the device may be flatly unable to produce laser oscillation at all, or, even if it can, its useful life is extremely short, making the device practically unusable. The development of such cracks is remarkable in a device structure including a layer containing Al, and, since a nitride semiconductor laser device typically includes such a layer, it is very important to eradicate cracks.